Rivets are probably the oldest and most widely used fasteners in the industry. They are inexpensive to manufacture, do not require close tolerance holes, and they are easily installed. Riveted structures usually have an extended service (fatigue) life because of the hole-filling characteristics of the rivet. When rivets are driven, the rivet shank expands and tends to fill the hole. This eliminates relative movement between the joined structural members, thus, providing the extended fatigue life.
Pins installed with an interference fit, which puts residual hoop tension stresses around the hole, are used to enhance the fatigue life of structures. These pins are expensive and require very close tolerance holes and, therefore, are expensive to install. One such fastener is the Hi-Tigue pin, manufactured by the Hi-Shear Corporation, Torrance, Calif.
It would be more desirable to place residual hoop compression stresses around the hole. One technique to accomplish this, which has been developed by the Fatigue Technology Corporation, Seattle, Wash., is to expand the hole with a removable sleeve and mandrel. The mandrel, while it is being pulled through the hole, stresses the material around the hole beyond the proportional limit in hoop tension. Thus, when the mandrel is removed, the material around the hole recovers elastically and forms residual hoop compression locally around the hole.
To obtain better control of the filling of the hole when driving, various rivet designs have been proposed and developed. For example, U.S. Pat. No. 1,966,401, "Rivet" by B. T. Andren. Andren discloses a protruding-head rivet wherein the underside of the head is tapered downward. Thus, when the rivet is driven, initially the material in the head will meet little resistance when flowing into the shank portion within the hole. Thereafter, the underside of the head becomes flattened and comes into contact with the material being joined and, thus, distributes driving loads uniformly into the structure. In this design there is no relationship between the taper angle and head shape or height so as to provide a "controlled" expansion of the rivet shank within the hole. Thus, there is a significant possibility of not filling the hole or overexpanding it and, additionally, trapping of sealant, if used, when the rivets are driven. Therefore, the relationship between the material in the head, the shape of the standard protruding-head rivet set, the degree of undercut of the head, and the head-to-shank radius are critical and must be tightly controlled.
Of additional interest in U.S. Pat. No. 3,561,102, "Process of Forming a Cold Driven Riveted Joint" by J. A. Diemer. Diemer discloses a protruding-head rivet having a circular head flattened at the top. In addition, the transition between the shank and the head is conical in shape and connects to the head in a large radius. While this design appears to be attempting to control expansion of the shank portion in the hole, specialized tooling is required for driving the rivet, which is a decided disadvantage.
In none of the above rivets has an attempt been made to make all critical dimensions of the head and tail, before and after driving of the rivet, a function of the shank diameter. While in most cases standard rivet sizes will suffice, there are instances where special diameters (switching from inch to metric sizes, etc.) are required. Thus, a rivet having all the critical dimensions based on the shank diameter would be advantageous, since design and test time required to insure necessary static tension, shear strength, and fatigue life would be reduced to a minimum.
Material selection for rivets is also a critical problem. If the material has a high-yield stress, difficulty if encountered when driving because of a tendency of the bucked tail portion of the rivet to crack and fatigue life will be decreased because of lack of hole filling. If the rivet material is too malleable, the rivet may not have sufficient strength. So the selection of material properties becomes a trade-off.
One of the ways around this problem has been the use of the 2024-T4 aluminum alloy "icebox" rivet material which, when solution heat-treated, quenched, and stored at a temperature below zero degrees Fahrenheit, remains in a relatively soft condition. Upon return to room temperature, this aluminum alloy rivet must be driven within 15 minutes since it quickly age hardens. Thus, the name "ice box" rivet. While this rivet is widely used, it is expensive to handle. If prematurely brought to room temperature, it cannot be driven without cracking the bucked tail. Rivets inadvertently allowed to harden prior to driving must be reheat-treated. Therefore, from the time of removal from the refrigerator and transport to the assembly station, the rivets are carried and stored in a container filled with dry ice. It is inevitable that some rivets will be driven after they are too hard; thus, they crack and must be drilled out and replaced. The added expense is obvious. Therefore, a considerable cost savings can be achieved, particularly in the manufacture of aircraft, if a non-icebox material that provides the same static strength and fatigue life as the icebox rivet can be obtained.
Additional patents of interest are as follows: U.S. Pat. No. 221,447, "Rivet" by J. B. Cornell; U.S. Pat. No. 3,821,871, "Fatigue Resistant Fasteners" by H. A. Schmitt, and U.S. Pat. No. 4,230,016, "Fatigue Resistant Fasteners and Method of Manufacturing Joints Therewith" by H. B. Merrell.
Therefore it is a primary object to provide a rivet that, when driven, obtains controlled expansion of the hole, placing residual hoop compression stresses in the hole wall and thereby increasing the fatigue life.
It is another primary object of the subject invention to provide a rivet wherein all critical dimensions thereof are ratios of the shank diameter.
Another primary object of the subject invention is to provide a 7050 aluminum alloy rivet that can be used directly as a substitute for the 2024 aluminum alloy icebox rivet.
A further object of the subject invention is to produce a rivet that can be installed and driven in holes produced with standard twist drills.
A still further object of the subject invention is to provide a protruding-head rivet that has a significantly lower-weight head than the Military Standard MS20470 protruding-head rivet while maintaining the same tension and shear strength for a given material.
Another object of the subject invention is to provide a rivet that can be insalled and driven with or without tank sealant or primer.
An additional object of the subject invention is to provide a rivet that can be driven with all standard protruding-head driving tools, whether hand bucked, squeezed, or automatically installed.